The present invention relates generally to signal pickup cartridges for reproducing signals recorded on rotary recording mediums. More particularly, the invention relates to a signal pickup cartridge in an apparatus for reproducing an information signal recorded on a rotary disc along a spiral or concentric track. The invention effectively suppresses the deflective resonance of a cantilever on the free end of which a signal reproducing element, such as a reproducing stylus, is mounted.
Heretofore, there have been developed apparatuses of the type wherein, for example, a rotary disc (referred to as "disc" hereinafter) has a video signal recorded on a spiral track as variations in the geometrical shapes corresponding to an information content. This spiral track is formed on a flat plane and has no stylus guide groove. A signal reproducing stylus of a signal pickup device traces the spiral track to reproduce the recorded video signal. In a signal pickup device of this character, it is necessary that the signal pickup device trace the track accurately, since the track has no stylus guide groove. For this reason, it is necessary to provide means for detecting any tracking deviation of the signal pickup device relative to the above mentioned track on the disc and for controlling, in response to this error, the position of the signal pickup device so that it will trace accurately over the track to accomplish a tracking control.
At the time of the production of a disc, mechanical deformation (distortion) therein cannot be avoided, and even when it is rotated at a constant speed at the time of its reproduction, variations arise in the relative speed between the recorded track and the reproducing stylus. This gives rise to jitter (error in the time axis) in the reproduced signal.
The present applicant has previously proposed a signal pickup device in which a permanent magnet member of the shape of a rectangular parallelepiped and having magnetic poles on the opposite lateral faces thereof is fixed to the proximal end of a cantilever. A first coil is disposed to surround the permanent magnet member. Second coils are disposed to confront each magnetic pole of the permanent magnet member. In this device, when the tracking control signal current and the jitter compensation signal current respectively flow through the first coil and the second coils, the permanent magnet member is energized to rotate about ahypothetic vertical axis thereof and to displace in the longitudinal direction thereof, according to Fleming's lefthand rule, whereby the cantilever is rotated and displaced in the axial direction of the cantilever. Tracking control and jitter compensation are thereby carried out.
This signal pickup device, however, is accompanied by following problems, due to its construction. The permanent magnet member, which is a moving member, must be miniaturized and light-weight, which inevitably causes a small magnetic force. Accordingly, the driving force of the permanent magnet member becomes small, whereby the operation of the cantilever with large driving force becomes difficult. Difficulties occur and the prescribed control operation cannot be performed.
Furthermore, since the reproducing stylus becomes worn with use over a long time, it is necessary that the cartridge be of an interchangeable construction wherein the cantilever carrying the reproducing stylus can be readily attached to or detached from a control drive part or an actuator part having coils.
The present applicant has developed a signal pickup device in which three coils are connected to the proximal part of a cantilever and are disposed within a magnetic field developed by a stationary permanent magnet member. Two coils among three coils are subjected to a torque when a tracking control signal current flows therethrough. The remaining coil is subjected to displacement in the winding axial direction when a jitter compensation signal current flows therethrough. The tracking control and jitter compensation are effectively carried out, as shown in detail in the U.S. Pat. No. 4,160,268.
The pickup cartridge of this previously developed pickup device incorporates a mechanism which presses the proximal part of a cantilever against a pivot bearing mounted to the moving coils to engage with the pivot bearing. A mechanism has a damper, which does transmit force at the time of jitter compensation. The mechanism applies a stylus pressure to hold a reproducing stylus against the disc.
In the reproducing operation of the above described signal pickup device, a vibration accompanying stick-slips produced during the sliding contact between the reproducing stylus and the disc and a vibration generated by the driving power imparted to the control driving part including coils are applied to the cantilever of the cartridge. For this reason, the cantilever undergoes deflective vibration at the natural resonance frequency of its deflective mode.
Because of this deflective vibration of the cantilever and because of the great mass of the cantilever and parts accessory thereto and for other reasons, the following-up characteristic of the cantilever with respect to the disc is poor at high frequencies such as, for example, above 1 KHz. Consequently, when the cantilever undergoes the above mentioned deflective vibration, the reproducing stylus tends to separate from the disc surface at a number of specific vibration frequencies. The envelope of the reproduced signal fluctuates in response to the contacting state of the reproducing stylus with respect to the disc surface.
More specifically, when the reproducing stylus separates slightly from the disc surface, the level of the reproduced signal becomes lower. In the case where the degree of separation of the reproducing stylus from the disc surface is great, a signal dropout occurs in the reproduced signal. The higher the frequency of the component of the information signal, the greater is the effect of this separation of the reproducing stylus from the disc surface. When there is a deterioration of the frequency characteristics due to inflection of the envelope of the reproduced signal, dropouts, and the like in this manner, the quality of the reproduced signal is markedly impaired.
Furthermore, when the reproducing stylus skips over the disc surface and repeatedly separates away from and drops onto the disc surface, scaly scuff marks are formed on the disc surface as a result of the impact imparted thereto each time the stylus drops onto the disc surface.
Still another problem arises in the case where minute undesirable convexities exist on the disc surface. In this case, the separation time of the reproducing stylus from the disc surface becomes longer because of these convexities. Furthermore, since the transient characteristic is not good, the stylus skips a number of times on the disc surface. In this case, also, undesirable results such as deterioration of the quality of the reproduced signal, dropouts, and damaging of the disc surface are incurred similarly as described above.
A further difficulty occurs when the cantilever is controlled and driven by the control drive part, at which time the cantilever is subjected to a vibromotive force, whereupon undesirable phenomena such as skipping of the reproducing stylus on the disc surfce and vibrational contacting and separating of the base or proximal end of the cantilever relative to the pivot bearing, which give rise to variation of the contacting state of the stylus with respect to the disc surface occur in some instances. These phenomena readily occur particularly in the case where the driving speed of the control drive part is high.
In particular, in the tracking control system, in order to compensate for phase delay of the drive system, a drive signal whose high-frequency band has been accentuated is supplied to the control drive part. For this reason, the stylus separates from the disc surface, and a dropout occurs in the reproduced signal at a specific frequency where a standing wave produces with a deflective vibration in the cantilever.
The above described phenomena readily occur particularly in cases where the reproducing stylus shifts from track to track within a short time at the time of a special reproducing mode. Examples of such special reproducing modes are the still-motion reproduction mode in which the same track on a disc on which a video signal is recorded is repeatedly reproduced, the high-speed or quick-motion reproduction mode in which the stylus reproduces by riding over one or a plurality of tracks, and the reverse reproduction mode in which the stylus reproduces as it rides over tracks in the reverse radial direction of the disc.
Accordingly, attempts have been made to suppress the deflective vibration of the cantilever by providing an annular rubber damper between an intermediate part of the cantilever and the base plate of the cartridge. The deflective vibration of a cantilever, however, has a relatively high frequency of, for example, 1 KHz or higher. For this reason, it is necessary that the elastic constant (i.e., stiffness S) of the rubber damper be large in order that it will function as an elastic member in the above mentioned frequency band.
However, in the case where a member having a great stiffness is used as a rubber damper, the contact force of the reproducing stylus against the disc surface when considered statically becomes a force far greater than a preferable force 50 mg because of this rubber damper, whereby there is the risk of the disc surface being damaged. On the other hand, if the stiffness of the rubber damper is so selected as to prevent the contact force of the stylus against the disc from becoming excessively greater than a desired value, the rubber damper will not function effectively as a damper at the above mentioned frequency band of the deflective vibration, and the provision of the rubber damper will merely be an addition of a surplus mass to the cantilever. Thus, the deflective vibration cannot be effectively suppressed by means of the above mentioned annular rubber damper.